1. Field of the Invention
This invention relates to thrust bearings for rotary cone rock bits that utilize fluid such as air to cool and clean the bit as it works in a borehole.
More specifically, this invention relates to a thrust bearing radially disposed between a roller cone and a journal and a means to secure and cool the thrust bearing therebetween. Still more particularly, the present invention comprises a thrust bearing made of very hard, temper resistant material, in which the non-wearing bearing is supported in the cone and channels are provided along the back of each thrust bearing for the purpose of allowing cooling air to contact each bearing.
2. Background
Rotary drill bits are generally well known in the art. These bits typically include three cone-shaped members adapted to connect to the lower end of a drill string. The bit includes three individual arms that extend downward from the bit body at an angle with respect to the bit axis. The lower end of each arm is shaped to form a spindle or bearing pin. A cone cutter is mounted on each spindle and adapted to rotate thereon. As the drill string rotates, the cones roll on the borehole bottom and rotate on about their respective spindles, thereby disintegrating the formation to advance the borehole. Individual bearing systems allow rotation of the cone cutter and serve to maintain the cone cutter on the spindle. These bearing systems have traditionally comprised roller bearings, ball bearings or friction bearings, or some combination of these.
In addition to providing bearings to facilitate rotation of the cone cutter, the interface between each spindle and its cone cutter typically includes a device for transmitting thrust (axial) forces from the cone cutter to the spindle and thence to the bit. These "thrust bearings" for rotary drill bits can generally be grouped into two different types. In one type the bearing system is sealed and the bearings are supplied with a lubricant. This bearing construction is most practical in oil drilling, where the axial feed forces are moderate. The frictional work and the development of heat in the bearings are small and the temperature in the bearings can be held at a low level. Thus, it is not necessary to supply air or other cooling fluid to the bearing. The other type of bearing is not sealed, and includes an internal channel or passage through which cooling air is supplied to the bearing. This type of bearing is commonly used in mining operations, where the load on the bearings is substantial. Because of the large load, the frictional forces on the bearing generate extreme heat and wear and it is necessary to provide a flow of air across the bearing to cool it, or an advanced material that has low friction characteristics and high temper resistance. The prior art includes several patents that disclose various wear-resistant means for transmitting the thrust load from each cone cutter to its respective spindle via a journal bearing surface and means for cooling the frictional interface.
For example, it is within the state of the art to provide a bearing surface, which comprises a hardmetal surface deposited on radially disposed areas of the journal. The deposited hardmetal material is subsequently machined to provide a smooth bearing surface between the journal and the rotary cone thrust button. This technique has the disadvantage that, when hardmetal material is metallurgically deposited within the cone or on the radially disposed bearing surfaces of the journal, the heat generated causes stress risers in the substrate that can result in cracks, especially in rotating cones. In addition, it is difficult, costly and time-consuming to machine the surface of these rough, hardmetal deposits to form the smooth bearing surfaces essential to the proper operation and longevity of the rock bit. These materials have poor thermal shock capabilities and limited resistance to the softening effects of high temperature.
Another type of thrust bearing comprises a pair of "thrust buttons" that are manufactured separately from the cone and spindle and then inserted into opposing recesses in the cone and spindle respectively. U.S. Pat. No. 4,194,794 to Kling discloses such a system, in which thrust buttons 22 and 25 are provided to transmit thrust loads from the cone cutter to the bit. Concentrating the load in a small area in this manner results in the generation of large amounts of heat as the frictionally engaged faces of thrust buttons 22 and 25 are forced to rotate with respect to each other. In addition, even small amounts of axial misalignment lead to severe stress concentrations at the edges of the buttons with this combination of materials because the hard, non-wearing members in the leg do not accommodate the misalignment. Kling further discloses an air cooling system that includes the placement of air ducts in each leg. The ducts are positioned so that they provide a flow of air at a point on the periphery of the spindle thrust button. The presence of dual roller bearings tends to reduce the cross-sectional spindle area available for air flow and thrust bearings.
Another variation on the double thrust button concept is disclosed in U.S. Pat. No. 4,098,358 to Klima. Klima discloses a spindle thrust button having a central opening and channels on its wear face that communicate with an air duct in the leg. The cone thrust button also has channels on its wear face. These channels communicate with the central opening and channels on the face of the spindle thrust button as the two buttons rotate relative to each other. While the placement of the cooling air channels on the wear faces of the thrust button would appear to allow more efficient removal of heat from the wear faces, this configuration actually produces a marked adverse effect. Namely, as a given portion of one wear face rotates over the other wear face, it is heated by friction with the other face. When the same portion then rotates over one of the cooling channels in the other wear face, it is subject to an abrupt reduction in temperature, which is then followed by another heat-generating contact, etc. This type of temperature fluctuation results in thermal fatigue that is very detrimental to high strength tool steels and hardmetal weld deposits. The surfaces of tool steel buttons, for example, develop fine heat checking cracks, which in turn eventually propagate until the button ultimately fails.
Still another disadvantage associated with the thrust buttons of the prior art arises out of the nature of the interference fit between each thrust button and the socket in which it is received. Specifically, while it is desirable to provide wide, robust teeth or knurls for engaging the hoop, it is also desirable to provide some clearance into which material deformed from the hoop can flow. As the amount of clearance area increases, the portion of the periphery of the thrust disc that is available for the teeth decreases. As the teeth are forced closer together, each tooth tends to get sharper. In addition, if it is desired to provide any flow of cooling fluid to the periphery of a thrust disc, the addition of passageways for the cooling fluid tends to further compromise the space available for the teeth and therefore tends to result in sharper, and therefore weaker, teeth.
Another common problem with the systems of the prior art lies in the relative positions of the wearing and non-wearing thrust bearing components. Specifically, if the non-wearing thrust bearing is placed on the spindle, the wearing member cannot compensate for misalignment of the friction bearing surfaces. Misalignment will occur after wear accumulates on the radial bearing and thrust bearing systems, with the result that the surfaces will no longer be parallel and a stress concentration will develop on the load side of the bearing.
Hence, it is desired to provide a thrust bearing system that adequately cools the thrust bearings without causing thermal fatigue, and compensates for slight misalignments of the cone cutter. It is further desired to provide a thrust bearing system for use in a rolling cone cutter that is relatively simple and inexpensive to manufacture and assemble, and that is durable under normal operating conditions. It is further desired to provide a thrust button configuration that allows for peripheral cooling of the thrust button and allows clearance into which deformed hoop material can flow, while still providing a plurality of wide, robust teeth, or knurls around the periphery of the disc.